Implanted pacing systems may be used to pace the heart. When the heart is paced in the left ventricle (LV), for example, there may be unwanted stimulation of the phrenic nerve that causes contraction of the diaphragm. Unintended phrenic nerve activation (an unintended action potential propagated in the phrenic nerve that causes a noticeable diaphragm contraction) is a well-known consequence of left ventricular pacing. The left phrenic nerve, for example, descends on the pericardium to penetrate the left part of the diaphragm. In most people, the left phrenic nerve runs close to the coronary vein targets for LV lead placement. The unintended phrenic nerve activation may cause the diaphragm to undesirably contract. Unintended phrenic nerve activation may feel like hiccups to the patient. Such unintended phrenic nerve activation can occur when the electric field of the LV pacing lead is proximate to the left phrenic nerve and is at a stimulation output that is strong enough to capture the nerve. Unintended phrenic nerve activation may vary from patient to patient. One reason for this variance is that the anatomic location of the phrenic nerve can vary within patients. Additionally, the veins in which the cardiac lead may be placed are not always in the same location with respect to the ventricle and the nearby passing nerve. Also, the selected position in which to place a cardiac lead for a prescribed cardiac therapy may vary.
Although phrenic nerve stimulation is commonly assessed at implant, unintended phrenic nerve activation caused by phrenic nerve capture during pacing may first appear or worsen post-implant for a variety of reasons. Therefore, special office visits after implant may be necessary or desirable to reprogram the device or worse, surgically reposition the lead to avoid phrenic nerve stimulation.